An LED (Light Emitting Diode) has a characteristic that a color tone thereof is changed depending on a current magnitude. Therefore, the LED is generally driven with a constant current. When performing a dimming control, the LED is turned on-and-off by a PWM signal, which is an external pulse signal, and a light quantity is adjusted by a duty ratio of the PWM signal.
In the meantime, when a switching power source is used as a constant-current power-supply device for driving a load with a constant current, it is necessary to perform a feedback control by detecting an output current. As described above, when the LED is driven by the PWM signal, the LED repeats a turning-on period and a turning-off period. During the turning-off period, the current does not flow through the LED, so that the zero output current is fed back. In this way, when the zero output current is fed back, the excess power is fed. In view of the above, a technology is disclosed that the feedback control performed only during the turning-on period of the LED to thus suppress the excess power from being fed (for example, refer to JP-A-2004-147435).
JP-A-2004-147435 discloses a non-isolated booster chopper-type switching power source. During an on-period in which the LED is turned on, an operating current is supplied based on an LED operating current indication value. During an off-period in which the LED is turned off, the LED and a supply voltage are separated by an n-type MOS transistor and a switching operation of the switching power source, which is an LED supply voltage source, is synchronously turned off. In this way, according to JP-A-2004-147435, the switching power source is turned on-and-off in synchronization with the PWM signal to reduce power consumption at a standby state.